Dimpled sheet

ABSTRACT

A dimpled sheet, with a flat area and a plurality of dimples protruding over the flat area, each having a dimple casing and a dimple cover, wherein contact surfaces for placement of the dimpled sheet against a subsurface are formed on the side of the dimples by the outer sides of the dimple covers. The proportion of the total contact surface of all dimples of the total surface of the dimpled sheet is greater than 35%, with the base surface of the dimpled sheet being formed from the flat area and the base surface of all dimples at the level of the flat area. The dimpled sheet has a protection coefficient (PC) of more than 10.000, whereby the PC is: 
         PC= ( CS ) 2   ×DH    
     with
         CS=total contact surface of all dimples in [%] of the total base surface of the dimpled sheet   DH=dimple height [min].

The invention relates to a dimpled sheet, particularly for construction purposes and preferably for use as foundation wall protection or drainage sheeting, with a flat area and a plurality of dimples protruding over the flat area, each having a dimple casing and a dimple cover, wherein contact surfaces for placement of the dimpled sheet against a subsurface are formed on the side of the dimples by the outer sides of the dimple covers.

Plastic dimpled sheets are used as foundation wall protection from surfaces in contact with soil. The known dimpled sheets consist of an impact-resistant and rigid plastic, particularly HDPE and PP, with the dimpled sheets lying flush in the installed state on the respective waterproofing and protecting them against mechanical damage during the filling of the pit by virtue of their cushioning characteristics. In the known dimpled sheets, in their installed state, the total contact surface of the dimples against the subsurface or against waterproofing is generally between 10 to 20% of the total surface of the dimpled sheet. Dimpled sheets of the type described in the foregoing are known, for example, as foundation wall protection sheets under the commercial name DELTA-MS or as drainage and protective sheets under the commercial name DELTA-GEO-DRAIN QUATTRO of the Dörrken Company.

Dimpled sheets with cup-shaped indentations in the form of truncated cones or truncated pyramids which serve to protect foundation walls are known, for example, from DE 33 02 244 A1. Moreover, a dimpled sheet is described in WO 82/03099 A1 wherein the stiffness of the dimples is increased through crater-shaped recesses in the center of the dimples. At the same time, very small, annular contact surfaces are formed by crater-shaped recesses on the dimple side of the dimpled sheet.

In higher-quality waterproofing materials made of bitumen, the punctiform transfer of the soil pressure via the dimples can have a negative effect on a bitumen sheet. As a result of the constantly acting soil pressure, the dimples sink with the contact surfaces into the 4 to 5 mm thick bitumen sheets. This can lead to damage to the seal. For this reason, dimpled sheets are put in place in part with the back side, i.e. with the side of the dimpled sheet facing away from the dimples, onto the waterproofing. It is disadvantageous here that the dimples facing outwardly toward the pit with respect to the building wall form a large contact surface for the soil to be filled in. There is the danger that, during the filling of the pit, the dimpled sheet will be torn at least partially from the waterproofing or will be shifted at least partially downward. In addition, the protective effect for the building is greatly reduced if the dimpled sheet is installed with the back side on the waterproofing, and the total contact surface of the dimples to the filled soil is merely 10 to 20% of the total surface of the dimpled sheet, so that stones impinging on the building wall and large particles in the filled soil can only be repelled to a small extent.

Moreover, with the back-side laying of the dimpled sheet on the waterproofing, it is disadvantageous that water transport cannot take place between the waterproofing and the dimpled sheet. Driving rain that flows down on the façade of a building wall can collect between the waterproofing and the dimpled sheet, which is a disadvantage. What is more, the heat-insulating effect of the dimpled sheet is reduced considerably with back-side installation compared to installation of the sheet with dimples facing the waterproofing. In this context, it also has to be considered that, during the laying of the dimpled sheet with dimples facing the waterproofing, a sufficiently large vapor pressure compensation layer is formed by the dimple gaps, hence permitting the transport of material by diffusion.

SUMMARY

It is the object of the present invention to make available a dimpled sheet of the type mentioned at the outset in which damage to the waterproofing cannot occur in the installed state in the event of installation with dimples facing inwardly towards the waterproofing, with the dimpled sheet also being intended to ensure a high level of insulation and a sufficient transport of material through the dimple gaps.

The aforementioned object is achieved in a dimpled sheet of the type mentioned at the outset by means of the features of claim 1.

The invention is based on the fundamental idea of enlarging the total contact surface of the dimpled sheet with the subsurface on the dimpled side such that damage to the waterproofing, particularly damage to bitumen sheets, through sinking-in of the dimples into the waterproofing under a soil pressure load is reduced considerably. Another contributing factor to this is, finally, that the contact surface between the dimpled sheet and the bitumen sheet is distributed relatively evenly by virtue of the inventive arrangement in order to reduce to the greatest extent possible excessive localized loads and a consequent sinking-in. Here it is especially advantageous if the total contact surface of the dimpled sheet is at least 45 to 50% of the total base area of the dimpled sheet. However, total contact surfaces between 50% and 70% are also very advantageous, whereby every single value within the interval between 36% and 70% is possible, that means 36%, 37%, . . . 69%, 70% including all decimal values within this interval. The total contact surface of the dimpled sheet is formed here by the contact surfaces of all dimples or by the areas of the dimple covers of all dimples of the sheet that come into contact with the subsurface during installation of the sheet according to the invention. In connection with the present invention, it has also been discovered that a sufficiently large vapor pressure compensation layer is produced even with the enlarged contact surface, so that sufficient transport of material is ensured.

To provide the protection of the sealing or waterproofing and to provide the drain or drainage as well as the areation function, the dimpled sheet is characterized in a protection coefficient PC of more than 10.000. Preferred are PC-values of more than 12.000, especially of more than 16.000 and more preferrably of more than 20.000, whereby every single value of more than 10.000 is possible (that means 10.001, 10.002 . . . ) including all decimal values, although if these values are not mentioned explicitly. The protection coefficient PC is defined by the formula

PC=(CS)² ×DH

with

-   -   CS=total contact surface of all dimples in [%] in relation to         the total base surface of the dimpled sheet     -   DH=dimple height in [min]

The total contact surface or area results from the sum of the upper surfaces or contact areas of the respective dimples over the total base surface or area of the dimpled sheet. The higher the total contact surface is the lower, is the sinking-in of the dimpled sheet in the ground and the better is the drain function. Further, the higher the dimples are, the better is the protection of the sheet against any soil or filling material like rocks, which might come in contact with the dimpled sheet. Surprisingly, it has been found out that the dimpled sheet has a very good function to solve the mentioned object if the protection coefficient PC has a value of more than 10.000 and especially in the range of the values mentioned before.

To prevent the dimple from collapsing under the soil load, a sufficient compressive strength per DIN 53454 is required. It has been observed in experiments that the compressive strength of the dimpled sheet or of the dimple should lie in the range of between 50 and 1500 kPa, and particularly between 150 and 1000 kPa. As will readily be understood, every intermediate value and every intermediate interval within the aforementioned limits is possible even if this is not mentioned in detail. In order to minimize the penetration of moisture through the sheet here, the dimpled sheet according to the invention should have a certain level of water vapor permeability. The sd value per EN 1931 should in any case be greater than 2 m, preferably greater than 50 m and especially preferably greater than 100 m. In order to have as little moisture penetration as possible, the sd value should be even greater than 500 m.

In order to be able to ensure sufficient drainage capacity as well as sufficient air compensation, the free volume between the dimples lies in the range between 500 to 10,000 mm³/m², preferably between 1000 and 7000 mm³/m². Especially preferred values lie between 1500 and 4000 mm³/m². Here, the free volume is measured between the flat area of the dimpled sheet and the level of the contact surface.

Moreover, the dimpled sheet can be embodied such that the following features are realized alone or in any combination with each other, namely

-   -   that the dimples at the level of the contact surfaces are         equally spaced apart from each other in all directions of the         dimpled sheet,     -   that dimples with respective square contact surfaces are         provided, with the edge length of the contact surface preferably         being approx. 11 mm and the distance between neighboring contact         surfaces being approx. 4 mm in all directions,     -   that dimples with respective rectangular contact surfaces are         provided,     -   that a plurality of rows of dimples arranged perpendicularly to         each other and having a plurality of dimples is provided, with         all dimples of a row of dimples preferably being arranged         axially to the middle longitudinal axis of the row of dimples,         and     -   that the dimples of a first row of dimples and the dimples of at         least one other neighboring row of dimples running parallel to         the first row of dimples are arranged in the direction of the         middle longitudinal axes of the neighboring rows of dimples in a         staggered manner with respect to each other.

When arranging the dimples, care should be taken that a minimum height be provided so that the air permeability is maintained on the dimple side even if the dimples should sink in slightly or be compressed by the soil pressure. With slope angles of the individual dimples of less than 90°, the distance between the dimples at the level of the dimple covers and the distance between the neighboring contact surfaces increases as the dimple height increases, which limits the reliable dimple height given the large total contact surface required. The dimple height should therefore preferably be 3 to 15 mm, particularly between 4 to 10 mm, more particularly 6, 8 and 10 mm. The wall thickness of the dimples can lie between 0.1 to 1.0 mm, particularly between 0.3 to 0.6 mm. A sufficient strength of the dimpled sheet can be ensured in this way.

In this connection it should be pointed out that the cross-sectional size and/or the cross-sectional shape of the dimples at the level of the dimple covers ends up determining the contour of the contact surface of a dimple. In this context, the dimple cover can have a square or circular or preferably level surface and an edge length or diameter of 3 to 30 mm. If smaller dimples and larger dimples are provided on the dimpled sheet, then the smaller dimples can particularly have an edge length or diameter of 3 to 15 mm and the larger dimples an edge length or diameter of 10 to 30 mm, in each case with respect to the contact surfaces of the individual dimples formed by the dimple covers.

Thermoforming or injection molding methods can be used to manufacture the dimpled sheet according to the invention. The dimpled sheet according to the invention can be manufactured from plastic, preferably from PE, PP, ABS, PS and/or PA and, optionally, at least one additive, particularly at least one thermostabilizer. The abovementioned materials are characterized by a high pressure load capacity as well as a high impact resistance. Moreover, the materials are resistant to bitumen and have a high stress crack resistance. If thermostabilizers are used as additives, the stress crack resistance and/or the resistance to ageing of the dimpled sheet according to the invention can be increased further.

With respect to information given in ranges and intervals above, in the following and in the claims, it goes without saying that every single value within an interval or range is comprised by the present invention and regarded as essential to the invention, even if concrete individual values are not mentioned in detail.

Particularly, there are a number of possibilities of embodying and modifying the dimpled sheet according to the invention, in light of which we refer to the dependent claims on the one hand and to the following description of preferred sample embodiments of the invention with reference to the drawing on the other hand. Moreover, the invention permits a combination as needed of the features disclosed and described in the following in the claims and/or on the basis of the drawing, even if this is not described in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section from a cross-sectional view of a dimpled sheet, and

FIGS. 2-23 show sections of top views of the contact surfaces of dimpled sheets according to the invention formed by the dimple covers of the dimples, each in schematic representation.

DETAILED DESCRIPTION

The dimpled sheet 1 has a flat area 2 and a plurality of dimples 5 protruding over the flat area 2 and each having a dimple casing 3 and a dimple cover 4. On the side of the dimples 5, contact surfaces for placement of the dimpled sheet 1 against a bitumen sheet 6 are formed by the outer side A of the dimple covers 4, with said bitumen sheet 6 being applied to the outside of a building wall 7. The outer side A is the outer contact surface of a dimple cover.

In the dimpled sheet 1 depicted in FIG. 1, the dimples 5 have a truncated cone shape and have a substantially level dimple cover 4. The dimples 5 can also have the shape of a truncated pyramid, a cone, a cylinder or a rectangle. As will readily be understood, the dimple cover 4 can also have recesses in order to increase the rigidity of a dimple 5. For example, in this connection, the dimple cover 4 can have a circular or annular or even an n-cornered contact surface, with the contact surface of a dimple 5 being formed by the surface components of the dimple cover 4 acting against the bitumen sheet 6.

In the dimpled sheet 1 depicted in FIG. 1, a provision is made that the proportion of the total contact surface that is formed by the dimple covers 4 of all dimples 5 lying against the bitumen wall 6 is greater than 35%, preferably between at least 45 to 50%, of the base surface of the dimpled sheet 1, with the base surface of the dimpled sheet 1 being formed by the flat area 2 and the base surface of all dimples 5 at the level of the flat area 2. In this connection, the flat area 2 describes the entire surface of the level areas of the dimpled sheet 1 arranged between the dimples 5. As a result, the dimpled sheet 1 illustrated in FIG. 1 has a relatively large total contact surface to the subsurface. This makes it possible to ensure that, after the filling of a construction pit with fill dirt 8, either no or only slight penetration of the dimples 5 into the bitumen sheet 6 can occur due to the fill pressure brought about by the fill dirt 8. The danger of damaging the bitumen sheet 6 is hence reduced considerably. At the same time, it is ensured by the inwardly oriented dimples 5 that the dimpled sheet 1 is torn off or partially shifted downward during filling of the fill dirt 8 into a pit (not depicted) adjacent to the building wall 7. Moreover, the removal of water through the hollow spaces 9 arranged between the dimples 5 is ensured by the inwardly oriented dimples 5, with the hollow spaces 9 forming air cushions which have a heat-insulating effect and permit vapor pressure compensation. In addition, the individual hollow spaces of the dimpled sheets are connected to each other, although it is also possible in principle that a plurality of hollow spaces that are separated from each other can be provided.

In FIGS. 2 to 23, contact surfaces of dimpled sheets 1 for one cross-section of the respective dimpled sheet 1 are respectively depicted for the sake of example, with the contact surfaces being formed by outer sides of dimple covers 4 of a plurality of dimples 5. The base or cross-sectional surfaces of the dimples 5 at the level of the flat area 2 are not shown. Here it is possible that the dimples 5 have the same cross-section over the entire dimple height, as is the case in cylindrical or rectangular dimples 5, for example. As will be readily understood, the dimples 5 can also have a cross-section which changes over the dimple height, as can be the case with truncated cone- or truncated pyramid-shaped dimples 5.

In the embodiment of a dimpled sheet 1 depicted in FIG. 2, the formed contact surfaces are preferably spaced equally apart from each other in all directions of the dimpled sheet 1. Here, according to FIG. 2, the dimples 5 have square contact surfaces, and the dimples 5 as such can be shaped in the form of a truncated pyramid or be rectangular. In dimples 5 with square contact surfaces, a proportion of the total contact surface of greater than 50% of the total surface of the dimpled sheet 1 is achieved, for example, if every dimple cover 4 has an edge length of approx. 11 mm, with a distance between neighboring dimple covers 4 of approx. 4 mm, respectively.

In the embodiment shown in FIG. 3, the contact surfaces of the dimples 5, in turn, have the same spacing from each other in all directions of the dimpled sheet 1 at least at the level of the dimple covers 4, with the dimples 5 having rectangular contact surfaces. In this way, the total contact surface can be increased further while maintaining the intermediate spacing of approx. 4 mm.

Moreover, in the dimpled sheet 1 depicted in FIG. 3, a plurality of rows of dimples 10, 11 are provided which are arranged perpendicularly to each other and each having a plurality of dimples 5, with the dimples 5 of a row of dimples 10, 11 being respectively arranged axially on the middle longitudinal axis of the respective row of dimples 10, 11. In the embodiment portrayed in FIG. 3, a raster- or lattice-like arrangement of the dimples 5 on the dimpled sheet 1 is further provided. By contrast, the embodiment shown in FIG. 4 is such that the dimples 5 of a first row of dimples 10 and the dimples 5 of at least one other adjacent row of dimples 10 a running parallel to the first row of dimples 10 are arranged in a staggered manner with respect to each other in the direction of the middle longitudinal axis of the neighboring row of dimples 10, 10 a.

In the dimpled sheets 1 depicted in FIG. 3 and FIG. 4, the longitudinal and transverse sides of the dimple covers 4 are oriented parallel to the longitudinal and transverse sides of the dimpled sheet 1. It goes without saying that it is of course also possible to embody the longitudinal and transverse sides of the dimple covers 4 and hence the contact surfaces of the dimpled sheet 1 such that they run perpendicularly to the longitudinal and transverse sides of the dimpled sheet 1.

Since, as the size of the dimples 5 or of the contact surfaces increases, their compressive strength decreases, dimples 5, 5 a, 5 b can be arranged regularly, particularly alternatingly, next to each other with different-sized contact surfaces and/or different shapes of the contact surfaces. This is shown, for example, in FIGS. 5 to 7. For example, a provision is made in FIG. 5 to arrange dimples 5 with larger contact surfaces and dimples 5 a with relatively smaller contact surfaces next to each other, with the dimples 5 each having dimple covers 4 with a square contact surface and the dimples 5 a each having dimple covers 4 with a rectangular contact surface. Here, the contact surface of a dimple 5 a is approx. 50% of the contact surface of a dimple 5.

In FIG. 6 and FIG. 7, other dimpled sheets 1 are depicted which show the possible combinations of larger dimples 5 with smaller dimples 5 a, 5 b. The illustrated embodiments show that dimples 5 can be respectively provided with a first contact surface and other dimples 5 a, 5 b can be respectively provided with another contact surface deviating from the first contact surface in order to enlarge the total contact surface of the dimpled sheet 1, with the dimples 5 and the other dimples 5 a, 5 b having contact surfaces of different shapes and/or sizes. As a result, two, three or even more types of dimple of different shapes and/or sizes can be provided in order to make available a comparatively larger total contact surface of the dimpled sheet 1 to the subsurface.

In principle, a total contact surface proportion of greater than 50% of the base surface of the dimpled sheet 1 can also be ensured with dimples 5 that have circular contact surfaces, with cylindrical dimples 5 having a higher load capacity than dimples 5 of an n-cornered shape. One possible sample embodiment of a dimpled sheet 1 with dimples 5 that have circular dimple covers 4 is shown in FIG. 8, for example. In the dimpled sheet 1 depicted in FIG. 8, the condition indicated below must preferably be met in order to ensure a sufficient total contact surface proportion of greater than 50%:

[π·d ²/4]/[(d+a ₁)·(d+a ₂)]>0,5

Even with dimples 5 with circular contact surfaces, an increase of the load capacity of the dimpled sheet 1 can be achieved by having larger dimples 5 arranged adjacent to smaller dimples 5 a, each with respect to the size of the contact surfaces or the size of the dimple covers 4. This is shown, for example, in FIGS. 9 and 10. As also follows from FIGS. 9 and 10, the larger dimples 5 can be arranged in a regular raster, with the other, smaller dimples 5 a being disposed in empty spaces of the raster.

Moreover, dimples 5 that have contact surfaces having one shape and/or size can have other dimples 5 a that have contact surfaces having another shape and/or size arranged peripherally in the manner of a frame. This is shown, for example, in FIGS. 9 and 11.

In the dimpled sheet 1 depicted in FIG. 11, dimples 5 with dimple covers 4 are provided which form square contact surfaces. The dimples 5 are arranged here in the manner of a frame about a dimple 5 a whose dimple cover 4 forms a circular contact surface. The combination of dimples 5, 5 a with different contact surfaces is advantageous, since dimples 5 with an n-cornered contact surface ensure a larger surface utilization and dimples 5 a with circular contact surface ensure a greater pressure load capacity. Further examples of embodiments of dimpled sheets 1 having dimples 5, 5 a, 5 b with contact surfaces of different sizes and/or shapes or contours are shown in FIGS. 12 to 20. In the dimpled sheet 1 depicted in FIG. 20, the condition indicated below must preferably be met in order to ensure a sufficient total contact surface proportion of greater than 50%:

[π·d ²/4+4·x ₁ x ₂]/[(x ₂+2·a ₁ +x ₁)·(x ₂+2·a ₂ +x ₁)]>0,5

Dimpled sheets 1 are shown in FIGS. 13 and 16, each with 3 different types of dimples, with the types of dimple differing in terms of the size and shape of the contact surfaces formed by the dimple covers 4.

In another embodiment of a dimpled sheet 1, a provision can be made that the contact surfaces of neighboring dimples 5 a, 5 b formed by the dimple covers 4 have contours that are complementary to each other, at least in sections. For example, according to FIG. 21, three different types of dimple are provided in dimples 5, 5 a, 5 b, with one dimple 5 b being framed by a circular contact surface of four dimples 5 a, each having contact surfaces with inwardly curved longitudinal sides 12. This results in an annular space 13 which is bounded toward the inside by the outer edge of the dimple cover 4 of the dimple 5 b and toward the outside by the inwardly curved longitudinal sides 12 of the dimple covers 4 of the neighboring dimples 5 a. By virtue of the mutually complementary contact surfaces of the dimples 5 a, 5 b, an even greater surface utilization can be ensured.

Another possibility for enlarging the total contact surface of the dimpled sheet 1 consists in providing dimples 5 with oval or ellipsoid contact surfaces, with the contact surfaces each preferably being narrowed in the middle area and the dimples 5 being arranged such that neighboring contact surfaces, in turn, have a complementary contour to each other at least in sections. This is shown, for example, in FIG. 22. A combination of the dimples 5 shown in FIG. 22 with other dimples 5 a having circular contact surfaces is also possible and contributes to the enlargement of the total contact surface. This is illustrated in FIG. 23.

It is not shown that the dimple covers 4 can also have an n-cornered shape, for example as a hexagon, octagon or dodecagon. Moreover, as will readily be understood, the dimple covers 4 of the dimples 5, 5 a, 5 b can also have recesses, resulting in annular contact surfaces, for example.

Finally, it should be pointed out that dimples with different extensions in the longitudinal and transverse direction can also be arranged such that they have reciprocal directional preferences, which is shown in FIGS. 6, 20 and 21, for example. By reciprocal directional preferences is meant, for example, that rectangular dimples can be arranged both standing and lying. This can also be the case, for example, in the embodiments shown in FIGS. 3, 4, 5, 7, 18, 19 and 23.

At a preferred embodiment of the present invention the total contact surface CS has 36% and the dimple height DH is 8,2 mm. With these values the protection coefficient PC has an amount of 10.627,2. 

1. Dimpled sheet, particularly for construction purposes and preferably for use as foundation wall protection or drainage sheeting, with a flat area and a plurality of dimples protruding over the flat area, each having a dimple casing and a dimple cover, wherein contact surfaces for placement of the dimpled sheet against a subsurface are formed on the side of the dimples by the outer sides of the dimple covers, wherein the proportion of the total contact surface of all dimples of the total surface of the dimpled sheet is greater than 35%, wherein the base surface of the dimpled sheet is formed from the flat area, having a protection coefficient (PC) of more than 10.000, whereby the protection coefficient (PC) is defined by the formula PC=(CS)² ×DH with CS=total contact surface of all dimples in [%] of the total base surface of the dimpled sheet DH=dimple height in [min]
 2. Dimpled sheet as set forth in claim 1, wherein the protection coefficient (PC) is more than 12.000 and especially more than 16.000,00.
 3. Dimpled sheet as set forth in claim 1, wherein a compressive strength per DIN 53454 of between 50 to 1500 kPa, preferably between 150 to 1000 kPa, and particularly between 300 to 700 kPa is provided.
 4. Dimpled sheet as set forth in claim 1, wherein an sd value per EN 1931 of greater than 2 m, preferably greater than 50 m and especially preferably greater than 100 m and particularly greater than 500 m is provided.
 5. Dimpled sheet as set forth in claim 1, wherein the total contact surface of all dimples is up to 80% of the base surface of the dimpled sheet, preferably between 40 and 70% and particularly between 45 and 60%.
 6. Dimpled sheet as set forth in claim 1, wherein the free volume between the dimples lies in the range between 500 to 10,000 mm³/m², preferably between 1000 to 7000 mm³/m² and particularly between 1500 and 4000 mm³/m².
 7. Dimpled sheet as set forth in claim 1, wherein dimples each with a first contact surface and other dimples each with another contact surface deviating from the first contact surface are provided and that the dimples and the other dimples have contact surfaces of different shapes and/or sizes.
 8. Dimpled sheet as set forth in claim 1, wherein the dimples and the other dimples are arranged alternatingly next to each other, in each case at least with respect to the level of the contact surfaces.
 9. Dimpled sheet as set forth in claim 1, wherein the dimples are arranged in a regular raster and that the other dimples are provided in empty spaces of the raster, in each case at least with respect to the level of the contact surfaces.
 10. Dimpled sheet as set forth in claim 1, wherein the dimples are framed by the other dimples, in each case at least with respect to the level of the contact surfaces.
 11. Dimpled sheet as set forth in claim 1, wherein the dimples with respectively circular contact surfaces and other dimples are each provided with an n-cornered contact surface.
 12. Dimpled sheet as set forth in claim 1, wherein the dimples and neighboring other dimples have contact surfaces which are complementary to each other at least in sections.
 13. Dimpled sheet as set forth in claim 1, wherein the dimples are each provided with an ellipsoid contact surface and that each contact surface preferably has a narrowing in its middle area.
 14. Dimpled sheet as set forth in claim 1, wherein the dimple height is between 3 to 15 mm, particularly between 3 to 10 mm, more particularly between 3 to 7 mm.
 15. Dimpled sheet as set forth in claim 1, wherein the contact surfaces have an edge length or a diameter of 3 to 30 mm.
 16. Dimpled sheet as set forth in claim 1, wherein the wall thickness of the dimples is between 0.1 to 1.0 mm, particularly between 0.3 to 0.6 mm. 